1. Field of the Invention
The present invention relates to a bonding method for bonding circuit electrodes of a display panel for use in a liquid crystal display, a plasma display, or the like, to electrodes on a printed board forming electronic circuitry for driving the display panel. The, present invention also relates to a bonding apparatus for use with the same method.
2. Description of the Background Art
A display panel for use in a liquid crystal display, a plasma display, or the like, is structured such that a display section and circuit electrodes, which receive signals for driving pixels of the display section, are formed on a glass substrate. There is a known method for connecting the circuit electrodes of the display panel to the electrodes on the printed board for outputting signals for driving the display panel. In this method, the display panel and the printed board are bonded together using an anisotropic conductive film (see, for example, Japanese Patent Laid-Open Publication No. 8-107268).
A conventional circuit electrode bonding method is described below with reference to FIGS. 11 and 12. FIG. 11 is a view illustrating a display panel 1 and a printed board 6 which are bonded together using an anisotropic conductive film 4. FIG. 12 is an enlarged view of a portion of the display panel 1 illustrated in FIG. 11.
The display panel 1 includes a glass substrate, a display section 1a for displaying an image, and sets of a plurality of circuit electrodes 2 for receiving signals for driving the display section 1a. The display section 1a and the sets of a plurality of circuit electrodes 2 are provided on the glass substrate, such that the sets of a plurality of circuit electrodes 2 are positioned on the periphery of the display section 1a. As illustrated in FIG. 12, there are bonding portions 8 each provided in an area between one of the printed boards 6 and a set of a plurality of circuit electrodes 2. The number of bonding portions 8 to be provided is the same as the number of printed boards 6 to be bonded to the display panel 1. Alignment marks 3a and 3b are provided on opposite sides of each of the bonding portions 8, and used for detecting the location of that bonding portion 8.
The display panel 1 and each of the printed boards 6 are bonded together by subjecting the anisotropic conductive film 4 to a thermocompression process. The thermocompression process imparts, to compressed portions, conductive properties in a through-plane direction (a thickness direction) and insulative properties in an in-plane direction (a length direction).
Each of the printed boards 6 is provided with a conductor 7 and alignment marks 5a and 5b. The alignment marks 5a and 5b are used for detecting the location of that printed board 6. Since each of the printed boards 6 is bonded to a corresponding one of the bonding portions 8 via the anisotropic conductive film 4, a set of the plurality of circuit electrodes 2 and the conductor 7 of that printed board 6 are electrically connected via the anisotropic film 4.
Next, a method for determining the size and an attaching location of the anisotropic conductive film 4 is described with reference to FIG. 12. In FIG. 12, sizes regarding the alignment marks 3a and 3b, the bonding portion 8, and the anisotropic conductive film 4 are indicated by alphabetic characters. Specifically, L indicates an interval between a set of alignment marks 3a and 3b, M indicates a length of the bonding portion 8, P indicates a pitch between adjacent bonding portions 8, X indicates a distance between an alignment mark 3a and its corresponding anisotropic conductive film 4, and N indicates a length of the anisotropic conductive film 4.
In order for the anisotropic conductive film 4 to cover the bonding portion 8, length N of the anisotropic conductive film 4 is set so as to be longer than length M of the bonding portion 8. A difference N−M in length between the anisotropic conductive film 4 and the bonding portion 8 is provided as a margin for allowing the anisotropic conductive film 4 to completely cover the bonding portion 8. Accordingly, by making the difference N−M larger, it is ensured that the bonding portion 8 is completely covered. However, when the difference N−M is too large, the anisotropic conductive film 4 comes into contact with the alignment marks 3a and/or 3b, and therefore a set of alignment marks 3a and 3b cannot be properly detected. Accordingly, length N of the anisotropic conductive film 4 should be set so as to be longer than length M of the bonding portion 8 and shorter than interval L between the set of alignment marks 3a and 3b. That is, a relationship M<N<L is satisfied.
By attaching the anisotropic conductive film 4 such that its center is positioned at a midpoint between the set of alignment marks 3a and 3b, it is made possible to minimize an undesirable possibility that the anisotropic conductive film 4 might come into contact with the alignment marks 3a and/or 3b. Accordingly, the location in which the anisotropic conductive film 4 is attached is set such that distance X between the anisotropic conductive film 4 and the alignment mark 3a is X=(L−N)+2.
Next, a conventional bonding apparatus for bonding the bonding portion 8 and the printed board 6 together is described with reference to FIG. 13. In FIG. 13, a bonding apparatus B includes: a designed size storing section 108; an input section 122; a control section 124; a film cutting section 202; a film attaching section 204; a printed board attaching section 206; a display panel transfer section 208; and a heating and pressurizing section 210.
The designed size storing section 108 stores designed sizes (Nt, Xt, Pt) regarding length N of the anisotropic conductive film 4, distance X between the alignment mark 3a and the anisotropic conductive film 4, and pitch P between bonding portions (hereinafter, referred to as the “bonding portion pitch P”). The film cutting section 202 reads length Nt of the anisotropic conductive film 4 from the designed size storing section 108, and cuts an anisotropic conductive film 4 to length Nt. The film attaching section 204 attaches the resultant anisotropic conductive film 4 onto a first piece of bonding portions 8 so as to be distanced by Xt from the alignment mark 3a. 
Upon completion of the attachment of the anisotropic conductive film 4 to the first piece of bonding portions 8, the printed board attaching section 206 uses a camera (not shown) to detect the alignment marks 3a and 3b, and attaches the first piece of bonding portions 8 to the printed board 6 such that the alignment marks 3a and 3b are aligned with alignment marks 5a and 5b, respectively, of the printed board 6 in a prescribed positional relationship.
Upon completion of the attachment of both the anisotropic conductive film 4 and the printed board 6 onto the first piece of bonding portions 8, the display panel transfer section 208 moves the display panel 1 by a distance of pitch Pt. Then, another operation is started to attach an anisotropic conductive film 4 and a printed board 6 to the next piece of bonding portions 8.
The heating and pressurizing section 210 pressurizes and heats the anisotropic conductive film 4, which is attached between the bonding portion 8 and the printed board 6 in the above-described manner, using a known method. As a result of this, the bonding portion 8 and the printed board 6 are bonded together, and therefore circuit electrodes 2 on which the bonding portion 8 is provided are electrically connected to the conductor 7 on the printed board 6.
In the step of attaching the anisotropic film 4 to the display panel 1, the temperature of the display panel 1 occasionally rises as high as the anisotropic film 4 and is thermally expanded. Accordingly, the actual distance L between alignment marks 3a and 3b, and the actual length M of the bonding portion 8 can be varied in accordance with variation in temperature and a thermal expansion coefficient of the display panel 1. However, there are variations in temperature distribution and composition among display panels 1, and therefore the actual distance between alignment marks 3a and 3b has an error ΔL′ with respect to the designed size L. In the case where the value of error ΔL′ is considerable, there occurs a malfunction such that the anisotropic conductive film contacts the alignment marks 3a and/or 3b, or is not sufficiently large as to cover the bonding portion 8.
Further, when an anisotropic conductive film 4 cut to a proper length at room temperature is attached to the display panel 1 having a high temperature, heat of the display panel 1 is conducted to the anisotropic conductive film 4, and therefore the anisotropic conductive film 4 is elongated by thermal expansion. Accordingly, in some cases, the length of the anisotropic conductive film 4 and the distance L between the alignment marks 3a and 3b, which are properly set at room temperature, can be varied, so that the anisotropic conductive film 4 after being attached may be in contact with the alignment marks 3a and/or 3b. 
Furthermore, even in the case where no malfunction is caused in the step of attaching the anisotropic conductive film 4 to the display panel 1 having a high temperature, the display panel 1 and the anisotropic conductive film 4 are shrunk when cooled down to room temperature. Accordingly, a difference (N−M) in length between the anisotropic conductive film 4 and the bonding portion 8 varies by difference C′ in length shrunk by temperature drop to the room temperature. Therefore, even if a positional relationship between the anisotropic conductive film 4 and the bonding portion 8 is properly set in an attaching operation, such a positional relationship varies during use. When length N of the anisotropic conductive film 4 becomes shorter than length M of the bonding portion 8, connections between the circuit electrodes 2 and the conductor 7 are not secured.
Further still, length N of the anisotropic conductive film 4 attached to the display panel 1 is varied due to variation in operation of a device for cutting the anisotropic conductive film 4 or due to deformation of the anisotropic conductive film 4 by pressure applied for attaching the anisotropic conductive film 4. The actual length of the anisotropic conductive film 4 attached to the bonding portion 8 has processing error D′ with respect to its designed size resulted from variation among step capabilities. Specifically, in the case where the processing error D′ is considerable, there occurs a malfunction such that the anisotropic conductive film 4 does not sufficiently cover the bonding portion 8, or the anisotropic conductive film 4 contacts the alignment marks 3a and/or 3b. 
Thus, even if the anisotropic film 4 is cut to a designed size for an environmental temperature in an attaching operation of a processing step, and attached in a prescribed location, it is not ensured that the anisotropic film 4 has a proper length and is attached in a proper location under a temperature environment during use.